The present invention relates to a steering apparatus of rack and pinion type more particularly, to a structure for applying a restoring force required for restoration from a turning state to a straight-ahead state to a rack shaft slidable in a direction of its axial length for steering.
Known as one type of an automobile steering system is a rack and pinion steering system which comprises a rack shaft extending laterally of a car body and connected with right and left operating wheels (usually front wheels) at the opposite ends and a pinion meshed with the rack shaft at an intermediate portion thereof and connected with a steering wheel so as to be associated in operation therewith.
The rack shaft is supported in a cylindrical housing extending laterally of the car body so as to be slidable in a direction of its axial length and is linked with the right and left operating wheels through respective link members (tie rods) at the opposite ends projecting toward both sides of the housing, so as to be slid in response to rotation of the pinion resulting from the control of the steering wheel. The sliding motion of the rack shaft is transferred to the right and left operating wheels to change direction of the operating wheels. The tie rods are connected with the ends of the rack shaft via joint means such as ball joints, to transfer power between the ends which move linearly along its axis with the sliding motion of the rack shaft and the operating wheels which turn at an uniaxially swinging motion.
In general, an automobile steering system comprises means for allowing the operating wheels changed in direction by control of the steering wheel to be restored to the straight-ahead state by the action of a reaction force from a road surface. The means allows the operating wheels to be restored from the turning state to the straight-ahead state when a driver lets go of the steering wheel during a car being in motion. This restoration of the operating wheels can be well realized by components of a steering mechanism including the steering wheel being arranged in proper alignment with each other as viewed in plan. However, in a front portion of a vehicle in which the components of the steering mechanism is arranged, components of a drive mechanism including an engine, a transmission and others are also arranged in general, which makes it difficult to realize the proper alignment of the components of the steering mechanism.
On the other hand, many automobiles in recent years are equipped with a power steering system to save labor required for the control of the steering wheel for steering so as to afford a driver a pleasant steering feeling. The power steering system is so structured that a steering-assist actuator arranged in the steering mechanism assists the steering with the power generated in according to the control of the steering wheel. The power steering system can be roughly divided into the hydraulic type using a hydraulic actuator as the steering-assist actuator and the electric power type using an electric motor as the steering-assist actuator.
In a case where the hydraulic power steering system is combined with the rack and pinion type steering wheel system, a piston fixedly mounted on the rack shaft is arranged in a cylinder chamber which is formed by an intermediate portion of the housing of the rack shaft being sealed in liquid tight, thereby constituting the steering-assist hydraulic cylinder. However, in this arrangement, sealing members for sealing the cylinder chamber in liquid tight are in elastic contact with the periphery of the rack shaft at the both ends thereof and act as resistance to the sliding motion of the rack shaft, thus disadvantageously hindering the above-mentioned restoration of the operating wheels to the straight-ahead state resulting from the sliding motion of the rack shaft.
To avoid these disadvantages, various arrangements using an elastic body such as a spring to assist the restoration of the operating wheels to the straight-ahead state have been proposed. It is rational that the resilient force to assist the restoration is applied directly to a member which hinders the restoration, and the rack and pinion type steering system commonly adopts an arrangement in which the resilient force is applied to the rack shaft, as disclosed by Japanese Utility Model Application Laid-Open No. 57-159577 (1982) and others.
FIG. 1 shows an example of such a conventional type steering system, which is so arranged that a coiled spring B is fitted in an opening 20 of a cylindrical housing 2 containing a rack shaft 1 on each side (only one side thereof is shown). With this arrangement, when the rack shaft 1 is slid in the direction indicated by a white arrow in FIG. 1, a part of the rack shaft 1 sliding into the housing 2, or rather an end face of the ball joint 5 for use in jointing with a tie rod 4, comes into elastic contact with the other end of the coiled spring B through a washer B' and contracts the coiled spring. When the control of the steering wheel is released in this state, the rack shaft 1 is pressed in the reverse direction by the spring force of the coiled spring B to assist the restoration to the straight-ahead state.
However, with this conventional type arrangement, since the coiled spring B built in the opening 20 of the housing 2 is so disposed as to encircle the rack shaft 1, and the rack shaft 1 moves inside of the coiled spring B to the nearly whole extent of its sliding in both rightward and leftward directions resulting from the steering, the rack shaft 1, when slid, cannot avoid contacting with the coiled spring B. As a result of this, scratches resulting from the contact with the coiled spring B may develop extensively on the outer periphery of the rack shaft 1.
In the arrangement in which a steering-assist hydraulic cylinder S is provided in the housing 2 to build a hydraulic power steering system, as shown in FIG. 1, this scratched 4 rack shaft 1 comes into contact with a seal member 6 for sealing the hydraulic cylinder S at the one side thereof, to cause disadvantage that the seal member 6 is damaged and the sealing effect is impaired.
Also, that may cause an additional disadvantage that at the opposite side of the housing 2 shown in FIG. 1, a rack tooth which is formed on the rack shaft 1 to be meshed with a pinion rotatable in according to the control of the steering wheel is damaged to cause impairment in engagement between the rack tooth and the pinion to hinder the smooth sliding of the rack shaft 1 and affect the smooth steering.